FIG. 8 shows an exemplary piezoelectric transformer of a prior art, e.g., a center-driving type piezoelectric transformer disclosed in Japanese Patent Laid-Open No.9-74236 which has a driving section located at a center portion thereof. As shown in FIG. 8, input electrodes 51 and 52 are placed on top and bottom surfaces of a center portion of a sheet or plate type, e.g., long-plate-shaped, piezoelectric member 50 when viewed from top, respectively, wherein the input electrodes 51 and 52 face with each other. The center portion of the piezoelectric member 50 disposed between the input electrodes 51 and 52 are polarized in a thickness direction, wherein the center portion forms a driving section of the piezoelectric transformer. Output electrodes 55 and 56 are formed on right and left sides of the piezoelectric member 50, respectively.
Right and left end portions of the piezoelectric member 50 are polarized in opposite longitudinal directions with each other, wherein the end portions form power generating sections of the piezoelectric transformer. A ratio of a length L2 of the driving section to a longitudinal length L1 of the piezoelectric member 50 in the longitudinal direction, i.e., L2/L1, ranges from 0.3 to 0.6. With this configuration, a larger voltage-rise ratio is obtained.
However, in the prior art piezoelectric transformer, no consideration is given to the thickness of the piezoelectric member incorporated therein. Experimental data clearly indicates that the thickness of the piezoelectric member considerably affects the efficiency, i.e., a ratio of an output power to an input power, thereof. It is preferable to have in a piezoelectric transformer an enhanced efficiency, an increased output power and a large voltage-rise ratio simultaneously.